(a) Field of the Invention
This invention relates to lens systems for cameras using Leica size films and more particularly to a compact camera lens system of a field angle of view of or more than 60 degrees, F-number of 4.0 and short overall length.
(b) Description of the Prior Art
A lens system for compact cameras using Leica size films is generally selected to have a field angle of view around 63 degrees. In order to make a lens system of this class so compact as to be of a telephoto ratio of or less than 1.0, it is necessary that the rear side principal point position of the lens system should be before the first surface of the lens system. In order to make so, it is considered to adopt a lens type used generally in a telephoto type lens system in which the front lens group is of a convergent lens system and the rear lens group is of a divergent lens system. However, generally in a telephoto type lens system, the field angle of view required to retain a favorable performance is so narrow as to be much smaller than the above described 63 degrees.
In order to form a lens system for the object of the present invention, it is necessary to well correct the aberrations to a field angle of view of 63 degrees, that is to say, it is important to correct the off-axial aberrations.
There is a lens system of U.S. patent application Ser. No. 197,743 filed on Oct. 16, 1980 (which shall be known as the invention of the prior application hereinafter) filed by Suzuki et al as a lens system solving the above problems.
If the lens system according to the invention of the prior application is used as it is for a lens system of the F/4.0 class, due to the restriction of the effective diameter of the first lens component, the peripheral light amount will relatively reduce. Therefore, the effective diameter of the first lens component must be made large. In order to prevent the reduction of the peripheral light amount while retaining the compactness and correcting the spherical aberration, it is considered to make the first lens component a little thicker or to make the refractive index higher to make the radius of curvature larger. the latter way is adopted in the present invention. That is to say, in the invention of the prior application, the refractive index of the first lens component is defined to be 1.45 to 1.63. However, it is found that, if the refractive index is made higher than 1.63, there will be a merit that the effective diameter will be able to be made larger and thereby not only the astigmatic difference but also the off-axial aberrations likely to be difficult to correct will be able to be well corrected by forming other lens components as well balanced and well balancing various aberrations.